Pompe disease (Glycogen storage disease type II; MIM 232300) is a classical lysosomal storage disorder that causes death in the first year of life for patients with the infantile form due to a progressive cardiomyopathy leading to cardiorespiratory failure. Myozyme (alglucosidase alpha, recombinant human acid alpha-glucosidase/GAA, rhGAA) was approved in April 2006 as a treatment for Pompe Disease. The availability of ERT with Myozyme has improved the outcome of most patients with Pompe disease; however, a subset of infantile Pompe patients responds poorly to ERT, subsequently dying from cardiorespiratory failure. Poorly-responding Pompe disease patients generally lack residual GAA expression and these patients are cross-reacting immune material-negative (CRIM-negative). The Aims of this proposal would evaluate these rare CRIM-negative Pompe disease patients on ERT +/- immune suppression, by enrolling them in a prospective/retrospective natural history study through the Lysosomal Disease Network. Subjects would be enrolled from the Network and data collected regarding a number of clinical endpoints to determine the natural history of this disorder. Enrollment will include patients for whom a poor clinical response to current therapy is predicted. Patients studied will be all infantile crossreacting immune material (CRIM)-negative Pompe disease patients on a clinical protocol starting enzyme replacement therapy (ERT) with or without immune suppression. They will be compared to data obtained from infantile CRIM-positive Pompe disease patients started on ERT at similar ages and also enrolled in the protocol monitoring history and outcome of the disease on ERT.